A charged particle accelerator using quadrupole electrodes can be represented, for instance, by an RFQ (radio-frequency quadrupole) ion accelerator.
FIG. 1 shows the construction of a conventional RFQ ion accelerator. Opposing surfaces of quadrupole electrodes 2, 4, 6, 8 are undulating. FIG. 2 illustrates the undulating condition in vertical cross section and in horizontal cross section. Namely, FIG. 2(a) is a vertical cross section, and FIG. 2(b) is a horizontal cross section. A high frequency is applied to these electrodes. The electrodes 4 and 8 assume the negative polarity when the electrodes 2 and 6 assume the positive polarity, and the electrodes 4 and 8 assume the positive polarity when the electrodes 2 and 6 assume the negative polarity. As shown in in FIG. 2, furthermore, the electrodes 2, 6 and the electrodes 4, 8 have protrusions and recesses that are deviated by 180.degree. in phase. Therefore, when the electrodes 2, 6 assume the positive polarity and the electrodes 4, 8 assume the negative polarity, an electric field is established in the axial direction on the center axis as shown in FIG. 2. When the polarities of voltages applied to the electrodes are reversed, the direction of electric field is reversed in FIG. 2. If now an ion enters into the electric field established in the quadrupole electrodes from the left in FIG. 2 at a speed and in phase such that it always receives an acceleration field directed toward the right, then the energy of the ion increases monotonously. Further, an ion which has entered in a phase in which it is decelerated at first, is bunched in the succeeding particles and is then accelerated monotonously. In the radio-frequency quadrupole as described above, the ions that enter in any r-f phase can finally be accelerated efficiently while utilizing the focusing produced by the r-f electric field in the vertical and horizontal directions. Therefore, a very high transmission factor is obtained.
In the conventional apparatus, an accelerating tube 10 forms an r-f cavity resonator together with the electrodes 2, 4, 6, 8. FIG. 3 is a section view of a conventional radio-frequency quadrupole, wherein reference numeral 18 denotes an r-f cable for supplying an r-f electric power to the cavity resonator. An end 20 of a central conductor forms a coupler in the form of a loop antenna. The resonance frequency of the resonator is determined by the geometrical size thereof, and cannot be changed. In the case of an H.sup.+ accelerator of 100 MHz, for instance, the length will be about 1.5 meters and the diameter will be about 0.5 meter. When other ion species are to be accelerated using this apparatus, it is necessary to increase the incident energy in proportion to the mass m to bring the incident speed to be the same as that of H.sup.+ ions, since the energy of an ion is given by eV=1/2 mv.sup.2 (where e denotes the electric charge of an electron, V denotes an ion acceleration voltage, m denotes the mass of an ion, and v denotes the speed of an ion). Therefore, the exit energy also increases in proportion to the mass m. Namely, with the radio-frequency quadrupole which accelerates H.sup.+ ions to 1 MeV, As.sup.+ can be accelerated to 75 MeV.
Such characteristics are not suited for implanting a variety of kinds of ions at various energies, that is required by an ion implanter.
From a different point of view, to realize an accelerator for exclusively accelerating As.sup.+ ions to 1 MeV relying upon the radio-frequency quadrupole, the length should be reduced to 1/75 while maintaining the frequency unchanged, or the frequency should be decreased to 1/75 while maintaining the length unchanged. In the former case, the distance between the recess and the protrusion on the electrode surfaces is reduced to 1/75, presenting a problem from the standpoint of machining the electrodes. In the latter case, the diameter of the accelerating tube must be increased by about 75 times to establish the resonance condition for the cavity resonator, which is not realistic.
In the conventional system of the construction shown in FIG. 1, inner walls of the accelerating tube are contaminated by the ion sputtering after being operated for extended periods of time. Namely, reduction in the Q-value of the cavity resonator makes it difficult to generate a voltage maintaining a predetermined high frequency.